Option to set contexts format in config.json

Description

The Docker CLI config file (config.json) allows the output format of many CLI commands to be customized (e.g., psFormat for listing containers). There is no options for customizing the output format of the docker context ls command.

Steps to reproduce the issue:

1. Run docker context ls --format 'table {{ .Name }}\t{{ .Description }}'.
2. Attempt to set this as the default format in config.json.

Describe the results you received:

There is no config option for setting the output format of the docker context ls command.

Describe the results you expected:

A config option like "contextsFormat" for setting the output format of the docker context ls command.

Additional information you deem important (e.g. issue happens only occasionally):

Config file options are defined in cli/config/configfile/file.go.

Output of docker version:

Client: Docker Engine - Community
 Version:           20.10.2
 API version:       1.41
 Go version:        go1.13.15
 Git commit:        2291f61
 Built:             Mon Dec 28 16:12:42 2020
 OS/Arch:           darwin/amd64
 Context:           default
 Experimental:      true

Output of docker info:

Not applicable.

Additional environment details (AWS, VirtualBox, physical, etc.):

Start with cli/config/configfile/file.go, where Docker CLI config options are defined, then trace the docker context ls command's formatting path. Add support for a contextsFormat configuration option matching the existing command-line format behavior, and verify that config.json controls the default output while --format still works.